Deliberate mistake: Many times throughout the show, a large bandage or makeup blot is visible on Lea Michele's right wrist, which is used to cover up her "I Believe" tattoo. This is most visible during "Don't Rain On My Parade" and her "Poker Face" and "For Good" duets.
Deliberate mistake: During almost every scene that involves the main actors in the hallway or in a classroom with a window to the hallway, there are people passing by at random intervals. Even if the scene takes place in the middle of class or after school, there are random extras wandering the halls. In real life, unless they had about 15 minutes between each class and every single club met after school every single day, there wouldn't be that many people in the halls.
Answer: The show is purposefully stylized and over the top. It is not designed to be an accurate, realistic representation of American high school. The idea that the glee club is so unpopular other students can literally throw garbage at them with impunity is an over-exaggeration of how glee club is considered "un-cool" compared to things like football. However, there is a second way of looking at it. The audience is simply never shown anyone getting into any trouble for throwing slushies. Just because we don't see it doesn't mean it never happened.
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